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Foreign Service

Last month Dan introduced you to the electric air injection system found in many late-model Volkswagens and Audis. This time he’ll explain how to diagnose the system when it stops working as designed.

Last month I described the basic operation of the air injection system used on popular Volkswagen and Audi products. This electrically powered setup, which first appeared in the late 1990s, is still in use today. This month I'll cover system diagnosis.

The most common reason to check this air injection system is the presence of DTC P0411. By the book, this trouble code means Secondary Air Injection System Low Flow or Incorrect Flow Detected. But in reality, this code could mean anything from a simple mechanical problem such as a leak to major electrical trouble. Sometimes, a failing system gives you a combination of both mechanical and electrical symptoms.

Always begin by verifying that the air injection pump runs. Let me repeat two important points from last month's column before I proceed: First, this pump runs for only 70 to 90 seconds after a cold start, so plan your test time accordingly. Second, the ECM controls a relay that's commonly labeled J299. This relay, in turn, controls current to the air injection pump.

There are several things you can try if the pump doesn't run after a cold start. Grabbing the proper scan tool is the easiest, fastest step. You see, some scan tools support VW's activated output tests, which enable you to energize actuators such as the J299 relay (screen capture in Fig. 1 below) and the N112 relay, which I'll discuss later. If this output test turns on the air injection pump, it tells you that the pump, relay and related wiring are all good. It suggests that the ECM isn't getting the proper inputs from sensors such as the ECT and IAT. Does this vehicle have any other relevant symptoms and/or trouble codes?

Suppose the scan tool command won't turn on the air pump. Check the appropriate fuse and fusible link for the system first, then the J299 relay. Generally, the J299 relay doesn't cause much trouble on this system. But when in doubt, bypass the heavy-current contacts of the relay with an appropriate jumper wire. If the air injection pump runs, then the relay itself or the relay-to-ECM circuit has failed.

Of course, trying to operate the air pump directly with a pair of stout, fused jumper wires bypasses the ECM and all the related wiring. Remember that a normal pump draws 22 to 24 amps.

It's not uncommon to encounter a blown fusible link in the air pump motor's power supply circuit. The most common cause of fusible link failure is excessive current draw due to a binding or seized pump motor. According to Jim Newkirk, a European vehicle diagnostic specialist at Identifix, it's not uncommon for moisture to accumulate inside these air injection pumps in colder climates. Newkirk explained that the moisture may cause premature pump bearing failure. In turn, failed bearings lead to motor dragging, binding and eventually a complete seizure. An air injection pump also will lock up if this moisture happens to freeze. Frozen moisture may even crack the pump housing.

Disconnect the hose from the air pump outlet and visually inspect the outlet port; rust inside indicates the presence of moisture. Or, turn on the pump for a moment with the outlet hose disconnected and see if it blows out water, Newkirk said.

Air Pump Noises

You may encounter two general types of air injection pump noises. One is an annoying rattle that occurs only when the pump is operating. An alert customer may describe a rattle that occurs for a brief time after a cold start and then disappears until the next cold start. This usually tells you that the air pump's rubber insulators, such as those shown in photo 1 on page 18, are broken.

The other condition is an air pump that either hisses or whistles. An illuminated Check Engine light and DTC P0411 accompany these noises. These are the telltale sounds of a leaking air injection pump. The customer may confuse you by complaining that the engine sounds louder than normal. Indeed, an external leak does make this air injection pump louder than normal. But this clue may not be very helpful to technicians who aren't familiar with the sound of a healthy air pump. Furthermore, the customer may not clarify that this “loud noise” occurs only briefly after a cold start.

If the air pump is accessible, feel around it with your hand for air leaks while a helper operates the pump with fused jumpers or a scan tool. Or remove the pump and visually inspect it, especially the seam we're pointing to in photo 2. You may find a seal literally hanging out of this seam! As bad as this looks, this problem is usually very repairable.

I realize that it's your prerogative to either repair or replace certain parts. Also, some customers deserve and appreciate a break on a larger-ticket job. That said, one reason to fix this dislocated seal is that the repair is usually very reliable. Another reason is that, according to my homework, list price for a new OE air injection pump may range from less than 400 bucks to more than 800.

Anyway, Don Stallman of Integrity Foreign Car Repair in Canton , OH , was the first VW specialist to show me this trick. A set of rivets holds together the upper and lower parts of the air pump assembly. Carefully grind off the rivet heads and remove the rivets closest to the dislocated seal (photo 3). Next, use a seal pick or similar tool to tuck the seal back into place—but be very careful and patient here! Replace those rivets with machine screws and nuts after the seal is back in place.

Plumbing Leaks

Leaks in the air injection plumbing are another potential cause of noise as well as the P0411 trouble code. Identifix's Newkirk noted that one easy way to find leaks is to operate the air injection pump with the pressure hose disconnected from the combination valve. Plug the end of this hose while the pump is running and you can usually hear a leak in the plumbing, he said.

I explained last month that the combination valve, which is commonly called the combi valve, is located on or near an exhaust manifold. On one family of these air injection systems, manifold vacuum operates the combi valve—just like vacuum opened an old-fashioned EGR valve. On the other family, air pressure alone from the air injection pump pops open the combi valve.

These VW/Audi air injection systems usually use semiflexible corrugated plastic tubes or hoses equipped with quick-release fittings. This corrugated tubing may crack due to a combination of heat and age, causing noise as well as the P0411 code. Some sources stressed that mishandling was a fairly common cause of plumbing leaks on this air injection system. When they see cracked or broken parts, the first thing they ask is, “Who touched the car last?”

For example, the system's air supply hose is often connected to the air cleaner box (photo 4 above). A technician who's careless and ham-handed could easily damage this supply hose during a supposedly routine air filter replacement.

Remember, this corrugated air injection plumbing is typically a captive OE part. What's more, it's often not in stock and is seldom inexpensive.

Solenoid & Combi Valve

The last two potential trouble spots are a solenoid and the combination valve. This ECM-controlled solenoid, which is commonly labeled N112, controls the vacuum supply to the vacuum- operated combination valve. Manifold vacuum is routed to one side of the N112 assembly. When it's time to operate the air injection system, the ECM energizes N112 by grounding it. After N112 is energized, vacuum should flow through it to the combi valve. The vacuum should open the valve, allowing secondary air to flow through into the exhaust system. You can liken the operation of the combi valve itself to that of an old, traditional vacuum- operated EGR valve; that is, apply vacuum and the valve opens.

Suppose the ECM set code P0411 but the air pump is operating and the rest of the air injection plumbing is okay. Let's say you disconnect the supply or pressure hose from the combi valve, then, when you turn on the pump with a scan tool or jumper wires, there's a powerful blast of air from this hose. At the same time, there are no abnormal noises elsewhere in the system. If these events take place, then a bad combi valve may be the cause of the DTC. In other words, the combi valve isn't opening, for some reason.

Try applying vacuum to the combi valve with a hand-held vacuum pump. Replace the valve if it won't hold vacuum. But if it does hold vacuum, a lack of control vacuum may be the reason the valve won't open. Tee a vacuum gauge into the vacuum supply hose right at the combi valve. When the air injection system operates after a cold start, you should see manifold vacuum at the combi valve vacuum port. If you don't, there are three likely possibilities—a basic vacuum circuit problem, a bad N112 solenoid or a solenoid-to-ECM circuit failure.

As I explained earlier, the ECM should energize N112 by grounding it when the air injection system operates. Of course, the proper scan tool can energize N112 via an activated output test (Fig. 2 on page 14). The test occurs with the ignition on, engine off. Apply vacuum to the inlet side of the N112 relay assembly and watch the gauge you just teed into the end of the vacuum circuit. Whether you successfully energize N112 with a scan tool command or a jumper wire, the component should pass vacuum on to that vacuum gauge. If you energize N112 and it won't pass vacuum, then the component is bad.

Suppose you've eliminated other possibilities of the DTC P0411 on the air injection system equipped with the second style of combi valve. Air pressure alone pops this valve open. When in doubt, sources state, disconnect this style combi valve from the exhaust system and turn on the air injection pump. Replace the combi valve if it doesn't pop open and pass air.